By ELLEN NAKASHIMA The Washington Post

Washington - President Barack Obama disagrees with an independent watchdog group's conclusion that the National Security Agency's once-secret program that collects billions of Americans' phone records is illegal, the White House said Thursday.

In a major speech outlining his agenda to reform NSA surveillance, Obama last week pressed for a plan to move the phone records out of the government's control. But, unlike the watchdog group, he did not call for an end to the program, known for the statute under which the government claims authority to run it: Section 215 of the USA Patriot Act.

"On the issue of (Section) 215, we simply disagree with the board's analysis on the legality of the program," White House spokesman Jay Carney said.

He cited two federal court rulings that upheld the program's constitutionality, as well as the findings of at least 15 judges on the Foreign Intelligence Surveillance Court over the past seven years that the program could continue under the statute.

"The administration believes that the program is lawful," he said.

The program's fate is unclear. Congress is debating whether to end it, federal courts are weighing its legality, and administration officials are searching for a way to salvage the NSA's ability to sift the phone records for terrorist connections without maintaining a massive database.

The 238-page report released Thursday by the Privacy and Civil Liberties Oversight Board - arguably the most extensive review of the Section 215 collection's legality and effectiveness - highlighted other issues involving the program, including a little-noticed feature that lets analysts search subsets of phone records without strict privacy safeguards.

That feature amounts to a "back-door search loophole" for Section 215, said Gregory Nojeim, senior counsel for the Center for Democracy & Technology.

The NSA collects call detail records from most major phone companies. It gathers numbers dialed, call times and lengths, but not actual conversations. Analysts query the database using numbers that are reasonably suspected to belong to terrorists, applying a technique called contact-chaining. All the numbers that pop up as contacts, including those two or three degrees removed from the initial number, are put into a "corporate store."

But anything in the storehouse may be searched "for valid foreign intelligence purposes, without the requirement that those searches use only" selection terms that have met the threshold of reasonable suspicion of a link to terrorism, the board noted. It also said that the NSA may apply "the full range" of analytic tradecraft to mine the records.

The privacy board estimated that the store could include more than 100 million numbers, and all the call records of 1.5 million people. "Rather than just looking at a person's (phone call) contacts," the government can combine all this data and mine it in a way that reveals all kinds of private information about individuals," said Patrick Toomey, a staff attorney with the American Civil Liberties Union.

In an interview with National Public Radio this month, then-NSA Deputy Director John Inglis, who retired on Jan. 10, said that in 2012, NSA analysts "actually touched" 6,000 phone numbers. "A million (numbers) is theoretically possible," he said. "But when you then consider what the actual implementation of that is, it's a much different answer."

Nonetheless, the privacy board recommended that the NSA be required to make a determination of reasonable suspicion before analysts query the corporate store.

The privacy board, in its report, also confirmed that the surveillance court, which reviews NSA's applications for domestic surveillance, did not render a legal opinion on the Section 215 bulk collection until last August - about seven years after the court first authorized it. The Justice Department has argued that the program was effectively "ratified" when Congress reauthorized it in 2010 and 2011.

"It's incredible that this program was allowed to operate for seven years without its legal basis being spelled out by the court," said Elizabeth Goitein, an analyst at the Brennan Center for Justice. "This wouldn't happen in a regular federal court."